Conventionally, metal foil-clad laminates produced by laminating metal foils such as copper foils to prepregs (i.e., resin-impregnated substrates) prepared by impregnating nonwoven or woven glass fabrics with thermosetting resin compositions such as epoxy resins and then drying the fabrics have had "undulations" which are a surface roughness of about 5 .mu.m. For this reason, such conventional metal foil-clad laminates have been apt to encounter problems, such as poor contacts between resists and the laminates, and etching defects, when finer printed-circuit patterns are to be formed.
As expedients for eliminating the above problems, it has been attempted to, for example, use a thin woven glass fabric as a prepreg to be bonded to a metal foil or use a prepreg having an increased resin content. However, such expedients have failed to substantially completely eliminate the "undulation" attributable to the fabrics.
On the other hand, it is known that an inorganic filler-containing prepreg is used in laminates or metal foil-clad laminates using woven or nonwoven fabric substrates and that such an inorganic filler can be suitably used in a varnish in an amount of about 5-70% by weight in terms of the content of the filler in the solid components of the varnish. It is also known that the use of an inorganic filler improves dimensional stability, strength, electrical characteristics, and other properties as disclosed in, for example, JP-B-63-65092, JP-A-1-235293, and JP-A-1-150543. (The terms "J-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application".) However, laminates having surface smoothness cannot be obtained by any techniques disclosed in the above references. In addition, the problems on drilling processability, heat resistance under moisture-absorbed conditions, metal-foil peel strength, and the like remain unimproved and, hence, the laminates obtained by the above techniques cannot be used as recent metal foil-clad laminates, on the surfaces of which electrical or electronic parts are mounted.
Furthermore, improvements of drilling processing conditions, use of a woven glass fabric which has undergone open filament treatment, and other expedients have been attempted to improve drilling processability. However, the attainable roughness of wall surfaces of drill holes, which conventionally is around 30 .mu.m, has not been improved significantly.